1. Field of the Invention
The present invention relates to a liquid crystal optical device constructed by combining a liquid crystal panel having a liquid crystal layer with a light source capable of emitting a plurality of different colors.
2. Description of the Related Art
Various methods have been proposed in the prior art to achieve a color display utilizing a phenomenon called successive additive color mixing by using a liquid crystal panel as a shutter with a light source (for example, an LED or a CRT) mounted behind it. Prior art literature relating to such methods includes, for example, “4. A Full-Color Field-Sequential Color Display,” presented by Philip Bos, Thomas Buzak, and Rolf Vatne at Eurodisplay '84, France, pp. 7-9, Sep. 18-20, 1984. The successive additive color mixing method, unlike methods that use color filters or the like with respective color segments provided at each pixel position of the liquid crystal panel, achieves color display by successively projecting colored lights by rapidly switching between different-colored light sources. For the liquid crystal panel used with this method, a structure equivalent to that of a monochrome liquid crystal panel can be used. The light source disposed behind the panel emits three colored lights, for example, R (red), G (green), and B (blue), each for a predetermined duration of time, and the respective colored lights are projected in sequence (for example, in the order of R, G, and B) in time division fashion. The liquid crystal panel is controlled to turn on or off each display pixel in a manner synchronized to the predetermined duration of time. The light transmission state of each of the R, G, and B colors is determined by turning on or off the pixel in the liquid crystal cell in accordance with the desired color information. As the time that each of the single colored lights is projected is very short, the human eye perceives the respective colors, not as individually separate colors, but as one color produced by mixing the respective colors.
Next, as one method for driving the liquid crystal panel, a time division driving method will be described. FIG. 1 is a diagram showing matrix electrodes. As shown in FIG. 1, scanning electrodes (X1, X2, X3, X4, . . . , Xn) and signal electrodes (Y1, Y2, Y3, . . . , Ym) are respectively formed on a pair of substrates. To drive the matrix array of pixels located at the intersections of the respective electrodes, a voltage is applied to the scanning electrodes in sequence and, in synchronism with the application of the scanning electrode voltage, a voltage waveform corresponding to the display state is applied from each signal electrode. The light transmission state of each pixel is determined by the sum of the voltage waveforms applied to the signal electrode and scanning electrode associated with the pixel, and the display state is thus written to the pixel. More specifically, to write to one pixel, the transmittance, i.e., the light transmission state, of the pixel is determined by the sum of the voltage waveform applied to the scanning electrode (Xn) and the voltage waveform applied to the signal electrode (Ym).
Various types of liquid crystals can be used for liquid crystal panels that achieve a color display by utilizing the phenomenon of successive additive color mixing. For example, antiferroelectric liquid crystals and ferroelectric liquid crystals exhibiting ferroelectric properties, as well as TN type liquid crystals and STN type liquid crystals, can be used. Among them, liquid crystals exhibiting ferroelectric properties, because of their fast response times, are preferred for use as the liquid crystal material when using different-colored light sources in accordance with the successive additive color mixing method. As a technique for applying such time-division light emitting sources to ferroelectric liquid crystal panels, the prior art discloses a driving method that switches the light emission from one color to the next in a plurality of frames (scanning periods) (for example, refer to Japanese Unexamined Patent Publication Nos. S63-85523 (FIG. 1) and S63-85524 (FIG. 1)).
Next, a detailed description will be given below of a driving method for a liquid crystal panel constructed using an antiferroelectric liquid crystal.
FIG. 2 is a schematic diagram showing the arrangement of polarizers in a liquid crystal panel constructed using an antiferroelectric liquid crystal. Between the polarizers 21a and 21b, whose polarization axes a′ and b′ are arranged in a crossed Nicol configuration, is placed a liquid crystal cell 22 in such a manner that the average long axis n of antiferroelectric liquid crystal molecules, when no electric field is applied, is oriented substantially parallel to the polarization axis of either one of the polarizers (in the diagram, the polarization axis a′) so that the liquid crystal cell is put in a non-transmissive state (closed state) when no voltage is applied and in a transmissive state (open state) when a voltage is applied. Alternatively, the arrangement may be made so that, when the antiferroelectric liquid crystal exhibits a first ferroelectric state or a second ferroelectric state to be described later, the long axis of the liquid crystal molecules is oriented parallel to the polarization axis of either one of the polarizers. In this arrangement, when the liquid crystal exhibits the ferroelectric state in which the long axis of the molecules is parallel to the polarization axis, the liquid crystal panel is put in the non-transmissive state, and when no voltage is applied, the liquid crystal panel is in the transmissive state. Either arrangement is possible, but the following description deals with the liquid crystal panel in which the polarization axis of one of the polarizers is oriented parallel to the average direction of the molecules in an antiferroelectric state when no voltage is applied.
When a voltage is applied across the thus arranged liquid crystal cell, its light transmittance varies with the applied voltage, describing a loop as plotted in the graph of FIG. 3. When a voltage of first polarity is applied, the voltage value at which the transmittance begins to change when the applied voltage is increased is denoted by V1, and the voltage value at which the transmittance reaches saturation is denoted by V2, while the voltage value at which the transmittance begins to drop when the applied voltage is decreased is denoted by V5; further, when a voltage of opposite polarity is applied, the voltage value at which the transmittance begins to change when the absolute value of the applied voltage is increased is denoted by V3, and the voltage value at which the transmittance reaches saturation is denoted by V4, while the voltage value at which the transmittance begins to change when the absolute value of the applied voltage is decreased is denoted by V6. As can be seen from FIG. 3, when the voltage value exceeds the threshold, the first ferroelectric state is selected, and when the voltage of the second polarity opposite to the first polarity is applied, the second ferroelectric state is selected; in these ferroelectric states, when the voltage value drops below a certain threshold, an antiferroelectric state is selected.
FIG. 4 shows driving voltage waveforms for driving the antiferroelectric liquid crystal panel in a time-division fashion. The electrodes are formed on the respective substrates as shown in FIG. 1. The voltage waveform applied to a scanning electrode (Xn), the voltage waveform applied to a signal electrode (Ym), and the sum of the voltage waveforms applied to the pixel (Anm) at the intersection of the electrodes are shown in FIG. 4. The amount of light transmission (T) of the pixel changes according to the sum voltage waveform of FIG. 4; ON(W) indicates the white display state which is the transmissive state, and OFF(B) indicates the black display state which is the non-transmissive state. The period during which the voltage is applied sequentially to all the scanning electrodes is the scanning period (frame period) and, in a reset period (Re), the liquid crystal is forced into a prescribed state, in the illustrated example, the antiferroelectric state. In the selection period (Se) that follows, when the first or the second ferroelectric state is selected, the liquid crystal is put in the ON(W) state, i.e., the transmissive state, while when the antiferroelectric state is selected in the selection period (Se), the liquid crystal is put in the OFF(B) state, i.e., the non-transmissive state; in the non-selection period (NSe) that follows, the temporal change of the selected state is controlled.
As described above, in the antiferroelectric liquid crystal panel, it is generally practiced to reset the antiferroelectric liquid crystal to the first or second ferroelectric state or the antiferroelectric state immediately before writing to the pixel. For example, in FIG. 4, the selection period (Se) is immediately preceded by the reset period (Re), and in this reset period, a voltage lower than the threshold voltage is applied to the pixel to reset it to the antiferroelectric state. In this way, by resetting the state of each pixel immediately before writing necessary information to the pixel, a good display can be produced with each pixel being unaffected by its previously written state.
Next, a ferroelectric liquid crystal panel will be described in detail. It is known that, generally, a ferroelectric liquid crystal molecule moves in such a manner as to rotate along the lateral surface of a cone (hereinafter called the “liquid crystal cone”) when an external force such as an electric field is applied. In a liquid crystal panel constructed by sandwiching a ferroelectric liquid crystal between a pair of substrates, the ferroelectric liquid crystal is controlled by the polarity of the applied voltage so that the liquid crystal molecules lie in one of two positions on the lateral surface of the liquid crystal cone. These two stable states of the ferroelectric liquid crystal are called the first ferroelectric state and the second ferroelectric state, respectively.
FIG. 5 shows one example of the arrangement of a ferroelectric liquid crystal panel constructed using a ferroelectric liquid crystal. A liquid crystal cell 22 formed by sandwiching the ferroelectric liquid crystal between a pair of substrates is placed between polarizers 21a and 21b whose polarization axes are arranged substantially at right angles to each other (crossed Nicol configuration), in such a manner that the polarization axis of either one of the polarizers is parallel to either the long axis n1 of the molecules in the first ferroelectric state or the long axis n2 of the molecules in the second ferroelectric state when no voltage is applied. In the example of FIG. 5, the polarizers are arranged so that the polarization axis a′ of the polarizer 21a is substantially parallel to the long axis direction n2 of the ferroelectric liquid crystal molecules in the second ferroelectric state.
In the polarizer arrangement shown in FIG. 5, when the ferroelectric liquid crystal is put in the ferroelectric state in which the long axis of the molecules is oriented parallel to the direction of the polarization axis of one of the polarizers (in the illustrated example, the second ferroelectric state), light does not pass through, and the ferroelectric liquid crystal panel therefore produces a black display (non-transmissive state). Depending on the polarity of the applied voltage, the ferroelectric liquid crystal is switched to the other ferroelectric state in which the long axis of the molecules is not made to coincide with the polarization axis of the polarizer; in this state, as the ferroelectric liquid crystal molecules tilt at a certain angle relative to the polarization axis, light from a backlight is transmitted therethrough and a white display can thus be produced (transmissive state in which the transmittance is high). In the illustrated example, the polarizers are arranged with the polarization axis of one of the polarizers oriented so as to coincide with the long axis direction of the liquid crystal molecules in the second ferroelectric state but, alternatively, the polarizers may be arranged so that the direction of the polarization axis coincides with the long axis direction n1 of the liquid crystal molecules in the first ferroelectric state. In that case, the black display state (non-transmissive state) can be produced in the first ferroelectric state, and the white display state (high-transmittance state) in the second ferroelectric state. Either arrangement can be employed in the present invention, but the following description is given by taking as an example the case where the arrangement shown FIG. 5 is employed.
FIG. 6 shows the relationship between the value of the voltage applied to the ferroelectric liquid crystal panel and the light transmittance of the ferroelectric liquid crystal panel. As shown in FIG. 6, when a voltage of first polarity (positive polarity) greater in magnitude than a certain value is applied to the ferroelectric liquid crystal, the ferroelectric liquid crystal exhibits the first ferroelectric state; in this state, light can pass through the ferroelectric liquid crystal panel and, hence, is in the high-transmittance state. Conversely, when a voltage of second polarity (negative polarity) greater in magnitude than a certain value is applied, the ferroelectric liquid crystal exhibits the second ferroelectric state, the non-transmissive state, in which no light is allowed to pass through. As can be seen from the figure, the light transmittance of the ferroelectric liquid crystal is maintained even when the applied voltage becomes 0 V; that is, the display state, once written, can be retained even after the externally applied voltage is removed.
FIG. 7 shows typical driving voltage waveforms for the ferroelectric liquid crystal panel having the polarizer arrangement shown in FIG. 5. The electrode arrangement is the same as that shown in FIG. 1. As shown, the amount of light (light transmittance) transmitted through one pixel in the ferroelectric liquid crystal panel changes according to the applied voltage; ON (W) designates the white display state in which the transmittance is high, and OFF (B) indicates the non-transmissive state, i.e., the black display state. The voltage applied to the pixel (Anm) in the ferroelectric liquid crystal panel can be expressed as a sum voltage waveform representing the sum of the scanning voltage waveforms applied to the scanning electrode (Xn) and the signal voltage waveform applied to the signal electrode (Ym).
The driving voltage waveform shown in FIG. 7 has one scanning period (frame period) in order to produce a display based on display data for one frame. Each frame period includes a selection period (Se) for selecting the display state based on the display data and a non-selection period (NSe) for holding the selected display state; here, the selection period is preceded by a reset period (Rs) for resetting, irrespective of the previously display state, the ferroelectric liquid crystal to one of the ferroelectric states before writing the next display data. In FIG. 7, a pulse of positive polarity for forcing the ferroelectric liquid crystal into the first ferroelectric state, i.e., the white display state (high-transmittance state), is applied in the first half of the reset period, and a pulse of negative polarity, for resetting the ferroelectric liquid crystal to the second ferroelectric state, i.e., the black display state (non-transmissive) state, is applied in the second half of the reset period. In this way, in the ferroelectric liquid crystal panel, in order to produce a good display, it is generally practiced to provide a reset period for switching the ferroelectric liquid crystal between the two ferroelectric states, irrespective of the immediately preceding display state, by applying pulses of opposite polarities.
As a grayscale display method for a ferroelectric liquid crystal panel having only two states, i.e., the first ferroelectric state and the second ferroelectric state, it is practiced to provide a voltage gradient within the same pixel and thus distribute threshold voltages within the same pixel, or to split each one pixel into a plurality of pixels and apply a voltage individually to each split pixel, thereby achieving a grayscale display based on the ratio between the area of the high-transmittance white display state and the area of the non-transmissive state.
When driving the liquid crystal panel by using the earlier described successive additive color mixing method, if the period from the time the light emitting device mounted as a light source behind the liquid crystal panel emits a certain color to the time it emits the next color is set as the scanning period, the scanning period must be made shorter than about 20 ms in order to prevent changes in the color of light emitted from the light source from being perceived as flicker by the human eye. In that case, when, for example, the response speed of the liquid crystal and the performance of the currently available liquid crystal materials are considered, if the number of scanning electrodes is 100 or larger, a voltage can be applied to each scanning electrode only once within the scanning period.
In the conventional time-division driving method, the selection period is provided in sequence starting from the first scanning electrode. When there are 100 scanning electrodes, for example, the number of scanning electrodes is large, and the location of the selection period for the endmost scanning electrode is delayed compared with that for the first scanning electrode. As a result, as the scanning progresses from the first scanning electrode toward the n-th scanning electrode, the amount of light transmission decreases. FIG. 8 is a diagram showing bar graphs, in which the vertical axis corresponds to the scanning electrode location and the horizontal axis represents the length of time that light is transmitted through the pixels on each scanning electrode in the white display state. That is, when producing, for example, a white display, within the time during which the same light is emitted the length of time that the light is allowed to transmit through the pixels differs from one scanning electrode to the next as shown in FIG. 8, and uniform brightness cannot be obtained over the entire screen. Further, if the light emission is switched from one color to the next in a plurality of frames as in the prior art, the number of times that the voltage is applied to each scanning electrode increases correspondingly, and flicker occurs.